This invention relates generally to flat panel display devices and particularly to a system for compensating for transfer characteristic variations of the electron guns of such a device.
U.S. Pat. No. 4,126,814 issued to F. J. Marlowe discloses a system for uniformly controlling the visual display brightness of a modular flat panel display device. The display device is divided into a plurality of channels, each of which contributes a portion of every horizontal line of the visual output. Such display devices are known in the art as modular display devices. The visual display of a modular device is acceptable only when the display brightness of every channel is uniform within the perception capability of the human eye.
Additionally, the display brightness of the modules must be uniform within approximately 1% to assure that the seams between the modules are invisible. In a modular display device every channel uses a different electron gun. The electron guns and their associated circuitry typically do not have uniform transfer characteristics and, therefore, the display brightness of the channels will vary along with the transfer characteristic variations. The Marlowe system addresses this problem by storing the brightness drive voltages required to produce 64 different predetermined levels of picture brightness for each of the electron guns. The stored drive voltages are selected from a voltage range which is divided into 256 equal incremental voltages. The electron gun is driven by the incremental voltages and the voltages produced on an electron collector are compared with a reference voltage. The incremental voltage which brings the collector voltage up to the reference voltage is then stored. This procedure is repeated for 64 reference voltages so that 64 of the 256 incremental drive voltages are stored. Accordingly, uniform brightness for all modules in the display is achieved because the brightness levels for all the modules are referenced to the same 64 reference voltages. However, because the electron gun transfer characteristics vary, the drive voltage range from which the 256 incremental voltages are selected must be sufficiently broad to include the electron gun with the greatest cut off voltage at the low end and the gun which requires the most drive to achieve maximum brightness at the high end. In many instances the electron gun characteristics are widely different so that the 256 incremental drive voltage range must be expanded to include both these guns and as a consequence brightness resolution is lost.
The instant invention is directed to a system for adjusting the stored drive voltages to the electron guns utilized in a modular display device so that the drive voltage range is the same for all the electron guns and a maximum number of the 256 incremental brightness drive voltages can be utilized for all of the electron guns.